Diamond has a wide bandgap of 5.47 eV and a very high dielectric breakdown electric field strength of 10 MV/cm. Its thermal conductivity is the highest among materials. Thus diamond may be advantageously used to construct high-power electronic devices.
Furthermore, diamond is the most advantageous as a high-speed electronic device among semiconductors because of high drift mobility and Johnson's figure of merit. The Johnson's figure of merit indicates a velocity of carrier migration in device, with a higher figure of merit corresponding to a higher migration velocity. For this reason, diamond is believed to be the ultimate semiconductor suitable for high-frequency/high-power electronic devices.
Attention is paid on multilayer laminates including a substrate overlaid with a diamond film or the like. Most single crystal diamonds currently used for the fabrication of diamond semiconductor are type Ib diamonds synthesized by the high pressure method. Since type Ib diamonds contain more nitrogen impurity and can only be produced in a size of the order of 5 mm squares, they find few applications.
By contrast, the chemical vapor deposition (CVD) method has the advantage that polycrystalline diamond can be produced as a diamond film of a large area having a diameter of about 6 inches (150 mm) and of high purity. The CVD method, however, is difficult to synthesize single crystal diamond, which is suitable for most electronic devices. This is because single crystal silicon is typically used as substrate in the art. Since silicon and diamond are largely different in lattice constant (a mismatch of 52.6%), it is very difficult to heteroepitaxially grow diamond on silicon substrates.
Efforts were made to overcome the problem. For example, Non-Patent Documents 1 and 2 report that a diamond film is effectively formed on an undercoat of platinum (Pt) or iridium (Ir) by the CVD method. At the present, the research on iridium has been most advanced. The method involves the steps of providing a substrate of single crystal MgO, heteroepitaxially growing an iridium film thereon, pretreating the Ir film surface, i.e., effecting bias treatment with hydrogen-diluted methane gas by a DC plasma enhanced CVD method, and depositing a diamond film on the pretreated iridium film. There are obtained diamonds having a size of submicron order at the early stage to several millimeters at the present. The thickness of a diamond portion is several microns to about 100 microns (μm). In Non-Patent Document 3, for example, deposition is continued for 8 hours until a diamond film of about 100 μm thick is obtained.
However, when diamond deposition is continued for several hours or more in a conventional DC plasma enhanced CVD apparatus, electric charge builds up on the substrate surface because the deposited diamond is an insulator. A foreign substance is also formed on the surface of the counter electrode, causing charge buildup. Such charge buildup functions to retard the growth rate of diamond and allows for frequent occurrence of sparks, causing to introduce defects and fissures in diamond.